In optical communications applications, optical transmitter modules are used to generate optical data signals, which are then transmitted over an optical waveguide, which is typically an optical fiber. An optical transmitter module includes a driver integrated circuit (IC) that receives an electrical data signal containing bits of data at its input, and produces, at its output, an electrical drive current signal. The electrical drive current signal is applied to a light source, such as, for example, a laser diode or light emitting diode (LED), which causes it to emit an optical data signal. An optics system (e.g., a lens) receives the optical data signal and couples the optical data signal into the end of an optical fiber, which then transmits the optical data signal over a network to some destination.
An optical receiver module includes a photo detector such as, for example, a photodiode, which receives an optical data signal transmitted over an optical fiber. An optics system (e.g., a lens) of the receiver module couples the optical data signal from the optical fiber end onto the photodiode. The photodiode converts the optical data signal into an electrical data signal. Electrical circuitry (e.g., amplifiers, filters, and clock and data recovery circuitry) of the receiver module conditions the electrical data signal and recovers the data bits.
Optical transmitter and optical receiver modules may be packaged separately, but are often packaged together in an optical transceiver module to provide a single package that has both transmit and receive functionality. A variety of optical transceiver modules are in use today. An optical transceiver module may have multiple transmit channels and multiple receive channels or a single transmit channel and a single receive channel. One common optical transceiver module design of the latter type is commonly referred to as a Fiber Optic Transceiver (FOT) module design.
A typical FOT module includes a metal leadframe that is secured to a molded housing. A printed circuit board (PCB) disposed within the molded housing has various electrical components mounted thereon, including one or more active optical elements (e.g., a laser diode and/or a photodiode) and one or more integrated circuits (ICs) (e.g., a laser diode or LED driver IC and/or a receiver IC). The electrical components have electrical contact pads that are electrically coupled via bond wires to the leads of the leadframe. The molded housing typically comprises a polymer material that is transparent to the primary wavelength of light of the FOT module. An optics system is formed in or secured to the molded housing. The end of at least one optical fiber is secured to the molded housing adjacent the optics system. The optics system optically couples light between the end of the optical fiber and the active optical element.
One of the disadvantages of FOT modules of the type described above is that the coefficients of thermal expansion (CTEs) of the metal leadframe and of the molded housing differ greatly. For example, the CTE of the molded housing may be 70 to 100 parts per million (ppm) per degree Celsius (ppm/° C.), whereas the CTE of the metal leadframe may be 17 ppm/° C. This large difference between the CTEs can result in movement of the leadframe and of the molded housing relative to one another during exposure of the FOT module to temperature variations. The highest stress levels between the leadframe and the molded housing typically occur during the solder reflow process, during which temperatures of around 260° C. are typically sustained for relatively long periods of time (e.g., 10 seconds). Consequently, delaminations may occur at the interface of the leadframe and the molded housing, which can cause the bond wires to break, resulting in a defective FOT module.
One possible solution to the delamination problem is to include glass filler nano particles in the material of which the molded housing is made to lower its CTE to more closely match the CTE of the metal leadframe. The disadvantage to this solution is that the inclusion of the nano particles in the housing material results in Rayleigh scattering of light as the light attempts to propagate through the molded housing. This scattering of the light reduces the transparency of the housing, and thus reduces the optical coupling efficiency of the FOT module. Therefore, this solution is only partially effective and is not suitable for use in all cases.
Another proposed solution to the delamination problem involves using a transistor outline (TO)-can design for the FOT module. This solution eliminates delaminations and broken bond wires, but is not easily realizable in FOT modules because it requires that the electrical pins of the module be bent by an angle of 90°.
Integrated circuits (IC) are typically implemented as dual in-line pin (DIP) packages. A DIP package typically includes a plastic molded housing secured to a copper leadframe. The plastic material of which the molded housing is made includes a glass filler material that lowers the CTE of the molded housing such that the difference between the CTE of the copper leadframe and that of the molded housing is relatively small. By making the difference between the CTEs of the copper leadframe and of the molded housing relatively small, the potential for delaminations occurring due to temperature fluxuations is relatively small. The inclusion of the glass filler material in the plastic material, however, makes the housing nontransparent.
Accordingly, a need exists for an FOT module that is manufactured in such a way that problems associated with delaminations and broken bond wires are avoided.